1. Field of the Invention
The present invention relates generally to a method for batch annealing austenitic stainless steels. More particularly, the present invention relates to the selection of alloy compositions, to the preparation of the stainless steel coils, and to the defining of appropriate annealing parameters in order to successfully perform batch annealing of austenitic stainless steels, including light to foil gauge stainless steels.
2. Description of the Prior Art
In the manufacture of flat-rolled stainless steel sheet and strip products, it is necessary to intermittently anneal or soften the material for further cold-rolling operations. It is also necessary to anneal the material at the finish gauge to render it suitable for fabrication (i.e., stamping, forming, etc.). Annealing is necessary because cold reduction elongates the grains of the stainless steel, greatly distorts the crystal lattice, and induces heavy internal stresses. The steel that results from the cold reduction process is typically very hard and has little ductility. The annealing process allows the cold-worked steel to recrystallize, and if the steel is held at the proper annealing temperature for a sufficient time, the structure of the annealed steel will again consist of undistorted lattices and the steel will again be soft and ductile.
Annealing techniques may be divided into two general categories: (a) batch operations, such as conventional box annealing; and (b) continuous operations. In the stainless steel industry, the softening of flat rolled sheet and strip products is most commonly accomplished through the use of continuous annealing lines.
The continuous annealing process involves unwinding the coil from a payoff reel and continuously feeding the coil into and pulling the coil through a furnace and then rewinding the coil on a take-up reel. The furnace is typically electric or gas fired. The steel strip, while traveling in the furnace, is typically heated to a temperature in the range of about 1800.degree. F. to about 2200.degree. F. in the case of austenitic alloys and to a temperature in the range of about 1400.degree. to about 1800.degree. F. for ferritic alloys. The annealing temperatures vary depending upon the particular alloy being annealed, as well as the alloy's intended end-use.
The demand for light to foil gauge (i.e, 20 mils or less) stainless steel strip products (hereinafter referred to as "light gauge stainless steels" or "light-gauge strip") has increased in the stainless steel industry in recent years. In fact, stainless steel strip/foil products having such light gauges are in demand and are included in the product lines of a number of steel producers.
Annealing light gauge stainless steels presents technical as well as economical problems to the stainless steel industry. For example, during the high temperature continuous annealing of light-gauge stainless steels in the temperature range of about 1800.degree. F. to about 2200.degree. F. for austenitic stainless steel alloys, the yield strength of the material is greatly reduced thus making the strip prone to breaking. The breakage of the light gauge strip can be frequent in the continuous annealing line furnaces and the subsequent downtime and material loss can be costly. Furthermore, the productivity with light gauge stainless steel strip is very low compared to that for conventional gauge products, since the productivity for the light-gauge strip becomes limited by the maximum line speed allowed by the continuous annealing lines. Adding additional continuous annealing lines to increase productivity would be costly. Thus, the operating costs associated with such light gauge stainless steel can be relatively high.
One potential alternative to continuous annealing of light-gauge stainless steel strip is batch annealing. However, batch annealing has not been utilized for stainless steel austenitic alloys. For stainless steels, batch annealing has been utilized mostly in connection with heat treatment, at about 1400.degree. F. to about 1600.degree. F., of ferritic grades at hot-rolled band and, to a lesser extent, at intermediate gauge to soften the material for further cold reduction.
Significant improvements have been made in batch annealing technology since the late 1970s. Such improvements have come through the introduction of 100% hydrogen atmosphere, high convection devices, improved furnace design, and modern computer controls. These improvements in the batch annealing technology have resulted in an increase of energy efficiency and improvement of heat transfer rates during both heating and cooling periods, thereby producing more uniform properties throughout the coil and reducing the process cycle time by more than 50% over older batch annealing operations. The above-mentioned improvements, together with alternative impeller materials, have resulted in maximum temperatures attainable in commercially available annealing furnaces of approximately 1650.degree. F. or more. However, with further modifications and advancements, temperatures of 1700.degree. or higher should be achievable.
As noted above, batch annealing has not been utilized in connection with austenitic stainless steel alloys in general for a number of reasons. For example, austenitic stainless steel alloys require higher annealing temperatures than existing batch annealing furnace equipment would be able to sustain. Also, at the cooling rates allowed by conventional batch annealing, carbides would precipitate on grain boundaries and cause a breakdown of corrosion properties, which are among the most critical properties in stainless steels. Moreover, at the temperatures required for annealing the austenitic alloys, it is likely that sticking or localized diffusion welding would develop between adjacent coil laps and damage the surface of the strip. At light gauges, the sticking can be so severe that the strip can actually tear or at least develop creases during rewinding.
In summary, some minimum annealing temperature is required for recrystalization of typical 200 series and 300 series stainless steel alloys. However, it is known in the industry that as the austenitic stainless steel alloys are heated, intergranular carbide precipitation begins at temperatures of about 900.degree. F. or more. At even higher temperatures, the carbides begin to dissolve, with relatively high temperatures required for typical alloys to achieve substantially complete carbide dissolution. For example, typical T-304 stainless steel has approximately 0.075% carbon by weight and requires during conventional line annealing an annealing temperature of approximately 1850.degree. F. to achieve substantially complete carbide dissolution. The required annealing temperature for typical T-201 stainless steel is generally similar. If the temperature required for substantially complete carbide dissolution is not reached, intergranular carbides can remain and make the alloys unusable. As a result, the industry has utilized annealing techniques for austenitic stainless steel alloys that achieve relatively high annealing temperatures in order to dissolve carbides and also that achieve sufficiently high cooling rates in order to prevent carbides from forming during cooling. Carbides that are not dissolved during annealing or that form during cooling can render the alloy unusable.
Even with advances in batch annealing technology, batch annealing furnaces typically reach less than 1700.degree. F., which is below the temperature necessary for the substantially complete dissolution of carbides to occur in typical austenitic stainless steel alloys.
Even if the temperature at 1800.degree. F. is reachable by further advances of batch annealing technology, the cooling rate of the stainless steel coils after annealing at 1800.degree. F. would not be fast enough in a batch annealing furnace to prevent intergranular carbide precipitation in typical austenitic stainless steel alloys. According to a Continuous Cooling Transformation diagram, published in "Handbook of Stainless Steels"--McGraw-Hill, Inc., 1977, for typical T-304 alloys with 0.075 percent carbon by weight, the maximum time allowed for the coil to cool from 1800.degree. F. to approximately 1250.degree. F. is about 200 seconds to prevent intergranular carbide precipitation. Typically, it would take approximately 15 to 20 hours for coils to cool from about 1800.degree. F. to approximately 1250.degree. F. in production-scale batch annealing furnaces, which is not fast enough to prevent intergranular carbide precipitation in typical austenitic stainless steels. Thus, the annealing technique generally utilized for austenitic stainless alloys is continuous annealing in which high annealing temperatures of about 1800.degree. F. to about 2200.degree. F. are typically reached, and the cooling, often assisted by air blasting, is fast enough to avoid intergranular carbide precipitation.
However, as noted above, the productivity of continuous annealing lines is limited by the maximum speed of the line. Further, the continuous annealing line incur additional drawbacks such as strip breakage due to the greatly reduced yield strength at these high temperatures. This is particularly acute when the material is in the form of light gauge austenitic stainless steel. Correction of these problems is costly and would further reduce productivity.
Therefore, there is a need in the stainless steel industry to develop methods of batch annealing austenitic stainless steel strip, particularly light-gauge strip, that will result in final material properties that are equivalent or superior to those produced on conventional continuous annealing lines. Such methods should avoid the drawbacks associated with the processing of light-gauge stainless steels on such conventional continuous annealing lines. Such methods should also, where possible, utilize existing furnace equipment. In addition, such methods should avoid the development of sticking or localized diffusion welding between adjacent laps of the coils.
There are several austenitic stainless steels well known in the industry that have relatively low amounts of carbon. Examples of such low carbon austenitic stainless steels include T-201L and T-304L. T-201L has the following chemical composition by weight percent: 0.03 max C, 16.0-18.0 Cr, 5.5-7.5 Mn, 0.25 max N, 3.5-5.5 Ni, 0.045 max P, 0.030 max S, and 0.75 max Si. T-304L has the following chemical composition by weight percent: 0.03 max C, 18.0-20.0 Cr, 2.0 max Mn, 8.0-11.0 Ni, 0.045 max P, 0.030 max S, and 1.0 max Si.
Accordingly, it is an object of the present invention to develop methods of batch annealing austenitic stainless steel coils that will render final material properties equivalent to or superior to those produced on conventional continuous annealing lines. It is a further object of the present invention to allow the methods of batch annealing austenitic stainless steel materials to be utilized in connection with light-gauge products in which surface damage, such as caused by sticking between adjacent laps of coil, is minimized. It is yet a further object of the present invention to lower production costs over conventional continuous annealing lines while avoiding the drawbacks associated with such conventional continuous annealing lines.